The present invention relates to semiconductor packaging, and, more particularly, to a package providing an electrically insulating heat dissipation path from an incorporated integrated circuit die.
A major objective of the present invention is to provide an economical electrically insulating heat dissipation path by using essentially the same materials and processes used in fabricating the body of the semiconductor package. The invention applies, although not exclusively, to ceramic pin-grid arrays which comprise a series of laminated ceramic layers with conductive metal strips providing signal paths along layers and conductive metal vias providing signal paths through respective layers.
Integrated circuit technology has progressed rapidly in providing increased circuit density on an integrated circuit die. The increase in circuit density is correlated with an increase in heat dissipation, particularly in bi-polar technology circuits. To avoid heat damage, it is important to provide a thermal path from the die to the exterior of the incorporating package.
One standard type of pin-grid array includes laminated square ceramic sheets, each sheet including centrally located square apertures which are aligned to define a cavity for an integrated circuit die. Typically, the outer square dimensions of the sheets or layers are equal, but the apertures can be different so that tiers are defined as the layers are stacked. The tiers can be used, for example, as the location of bonding pads for electrically interconnecting an incorporated integrated circuit die to the signal paths of the package itself.
The package bonding pads are electrically connected to pins of the package by conductive strips formed along layers and conductive vias formed through one of more layers. The conductive strips can be formed of gold or aluminum. The vias can be of a refractory metal such as tungsten or an alloy thereof to withstand the temperatures used in laminating the ceramic structure.
To facilitate the removal of heat from the die cavity defined by the apertured layers, a heat spreader can be disposed at the base of the cavity. Heat removal can be achieved by arranging the heat spreader as the base of the die cavity and bonding the substrate of a die directly to the heat spreader. Additional heat transfer capability can be obtained by attaching a heat sink to the heat spreader, as described in an application for U.S. patent, Ser. No. 848,358. The heat spreader is generally a refractory metal, such as a copper/tungsten alloy. The heat sink, which can be attached after formation of the package itself, can be of aluminum.
In the foregoing arrangement in the thermal path is also an electrical path, which a design advantage in many applications, and is of little significance in others. However, there are applications in which it is necessary or desirable to have the die and the heat spreader electrically isolated. Otherwise, the die and its performance are rendered vulnerable to ambient electrical fields, as well as unplanned contact with neighboring wires and components once the package is integrated into a system. The incorporating system would thus be susceptible to intermittent or permanent disruption that could be difficult to diagnose. Thus, in most cases, it is desirable to insulate the die electrically from the heat spreader and heat sink.
Electrical insulation can be provided between the heat spreader and the die cavity by imposing a ceramic layer between the two. The ceramic layer then forms the cavity base to which the die substrate is attached. The disadvantage of this approach is that the ceramic is a poor conductor of heat, so excessive heat can accumulate in the die cavity.
Another approach would be to use, instead of a ceramic layer, a layer of thermally conductive and electrically insulating material between the heat spreader and the die. Boron nitride was the electrically insulating and thermally conductive material of choice in a very different context of a rectifier package in U.S. Pat. No. 3,728,584.
In the context of ceramic integrated circuit packages, however, such a material would have to be compatible with the processing and operating constraints of the ceramic package. It would have to withstand the elevated temperatures used in forming the package itself. In addition, the thermal expansion of the material must suitably match that of the ceramic. The material would have to be suitable for attachment to the ceramic layers, the die and the heat spreader. In addition to the problem of finding a suitable electrically insulating and thermally conductive material, the costs involved in obtaining and integrating the material into the package fabrication process must be considered.
What is needed is an integrated circuit package with an economical and practical electrically insulating thermal path from the die cavity to the package exterior. Preferably, the materials used in defining this path are substantially those used in fabricating the remainder of the package.